Silver halide emulsions used in silver halide photographic materials are generally subjected to chemical sensitization by various chemical substances to obtain desired sensitivity and gradation levels. Sulfur sensitization, selenium sensitization, noble metal sensitization (for example, gold sensitization), reduction sensitization and combinations thereof are typical sensitization processes.
In recent years, high sensitivity, excellent graininess, high sharpness and rapid processing having an increased rate of development have been needed for the silver halide photographic materials. Accordingly, the above-described sensitizing processes have been improved.
Sensitization is disclosed in U.S. Pat. Nos. 1,574,944, 1,602,592, 1,623,499, 3,297,446, 3,297,447, 3,320,069, 3,408,196, 3,408,197, 3,442,653, 3,420,670 and 3,591,385, French Patents 2,093,038 and 2,093,209, JP-B-52-34491 (the term "JP-B" as used therein means an "examined Japanese patent publication"), JP-B-52-34492, JP-B-53-295, JP-B-57-22090, JP-A-59-180536 (the term "JP-A" as used herein means an "unexamined published Japanese patent application), JP-A-59-185330,JP-A-59-181337,JP-A-59-187338,JP-A-59-192241, JP-A-60-150046, JP-A-60-151637, JP-A-61-246738, British Patents 255846 and 861984, and H. E. Spencer et al., Journal of Photographic Science 31, 158-169 (1983).
Although the selenium sensitization is generally higher in sensitizing effect than the sulfur sensitization more commonly employed in the art, it produces more fogging and tends to lower the contrast. Many of the above-described known patents attempt to overcome such disadvantages, but only insufficient results have been achieved. In particular, a basic improvement to depress the generation of fogging has been eagerly desired.
Further, when sulfur sensitization or the selenium sensitization is used in combination with gold sensitization, the sensitivity is significantly increased. However, fogging also increases at the same time. Compared to gold-sulfur sensitization, gold-selenium sensitization particularly increases fogging. For this reason, the development of techniques to depress the generation of fogging, particularly the development of selenium sensitizers generating less fogging, has been strongly desired.